Fabrication of Hybrid Electrodes by Laser-Induced Forward Transfer for the Detection of Cu2+ Ions

Composites based on poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS)—graphene oxide (GO) are increasingly considered for sensing applications. In this work we aim at patterning and prototyping microscale geometries of PEDOT:PSS: GO composites for the modification of commercially available electrochemical sensors. Here, we demonstrate the laser-induced forward transfer of PEDOT:PSS: GO composites, a remarkably simple procedure that allows for the fast and clean transfer of materials with high resolution for a wide range of laser fluences (450–750 mJ/cm2). We show that it is possible to transfer PEDOT:PSS: GO composites at different ratios (i.e., 25:75 %wt and 50:50 %wt) onto flexible screen-printed electrodes. Furthermore, when testing the functionality of the PEDOT:PSS: GO modified electrodes via LIFT, we could see that both the PEDOT:PSS: GO ratio as well as the addition of an intermediate release layer in the LIFT process plays an important role in the electrochemical response. In particular, the ratio of the oxidation peak current to the reduction peak current is almost twice as high for the sensor with a 50:50 %et PEDOT:PSS: GO pixel. This direct transfer methodology provides a path forward for the prototyping and production of polymer: graphene oxide composite based devices.


Introduction
Quantifying biological or biochemical processes is of utmost importance for medical, biological, and biotechnological applications. However, converting biological information to an easily processed electronic signal is challenging due to the complexity of connecting an electronic device directly to a biological environment. Electrochemical sensors provide attractive means to analyze the content of a biological sample due to the direct conversion of a biological event to an electronic signal. Today, many sensors can be found in labs around the world, and a growing number are used as diagnostic tools in point-of-care testing [1].
Like many other micropollutants, heavy metals represent a growing environmental [2] and health [3] problem. Depending on the contamination pathway, they appear at detectable levels in food resources, thus contaminating humans [4]. Some heavy metals, such as copper (Cu), are known to play a vital role in physiological concentrations but can also be toxic in larger doses, i.e., Cu affects the liver and kidney [5]. In addition, copper ions are severely hazardous environmental pollutants with a toxic effect on living organisms due to their participation in producing reactive oxygen species [6]. According to the World Health Organization and European water quality standards, the concentration of copper in drinking water should not exceed 2 mg/L [7].
Heavy metal trace detection is mainly carried out using spectroscopic techniques: atomic absorption spectroscopy [8], inductively coupled plasma mass spectroscopy [9], X-ray fluorescence, quartz crystal microbalance [10], and neutron activation analysis are used in electronic devices is highly critical, with very few defects and oxygenated groups. With cost-effective processing, graphene oxide (GO) sheets can be obtained directly from graphite after oxidation or exfoliation without undergoing further reduction. Given its large specific surface area and strong hydrophilicity, GO shows tremendous promise for removing aqueous pollutants, including heavy metal ions. In addition to serving as an excellent adsorbent, Cu 2+ binds strongly to oxygen moieties on the GO surface, enhancing its electrical conductivity while also improving its chemical stability.
The electrodes presented above have significant merits including high stability and reproducibility; however, their sometimes-inconvenient fabrication and high production costs may limit their application [35].
To date, few studies related to the use of PEDOT:PSS with graphene nanomaterials have been reported. The results obtained have shown that GO/PEDOT:PSS composites are promising candidates for modifying electrode material used in electrochemical sensing [36,37].
In this work we propose an original, flexible sensor based on a PEDOT: graphene oxide composite developed by a laser-based method, i.e., laser-induced forward transfer (LIFT), which has a low cost and can be successfully scaled. LIFT is a non-contact technique that may be used both for liquid-phase and solid-phase transfer [38,39]. The technique is suitable for applications where high spatial resolution is required, it doesn't require complex masks, as in lithography, and where it is not limited by the rheological properties of the material under transfer, providing the ability to print materials within a broad range of viscosities ranging from low viscosity Newtonian fluids (water) to non-Newtonian fluids (pastes). It involves two substrates, i.e., the donor substrate coated with the material of interest and a receiver substrate, which are brought in close proximity (<10 µm). During LIFT, a laser beam is imaged at the interface between the transparent laser support and the material to transfer (also named donor). Generally, the mechanism for solid-phase printing involves a single laser pulse that irradiates the donor substrate, and the thermally induced stresses introduced on the donor surface lead to the ejection of the solid flyer mimicking the shape of the laser spot [40]. Recently, LIFT has been applied for printing Au voxels as porous electrodes. The authors reported that the LIFT-ed electrodes presented an increase in the electrochemically active surface area by a factor of four compared with a sputtered dense Au film when characterized using cyclic voltammetry in Ar-saturated 0.1 M KOH [41]. In addition, in [42], a screen-printed electrode modified by LIFT of different liquid mixtures was demonstrated for the detection of organophosphorous and carbamate pesticides.
Moreover, in order to improve the process efficiency and to protect the material to be transferred, a sacrificial layer, called dynamic release layer (DRL), which is a metal or polymer film of tens of nanometers in thickness, is introduced between the donor substrate and the material of interest to absorb the laser energy and induce the transfer process.
In this work, we carry out transfers with and without a triazene polymer (TP) layer as a DRL [43]. Triazene is highly photoactive and strongly absorbs in the UV spectrum with a maximum absorbance of around 200 nm, making it a good candidate for a release layer. In this work, the TP layer is placed between the quartz substrate and the PEDOT:PSS: GO thin layer. Upon laser interaction, the TP layer decomposes, allowing the transfer of the PEDOT:PSS: GO layer onto a substrate (i.e., flexible screen printed electrodes) placed parallel and in close proximity to the donor. Our work seeks to demonstrate the applicability of laser-based methods for the realization of proof of concept cost effective sensors based on polymer: graphene oxide composites for copper ion monitoring, thus paving the way for innovative and affordable cooper ions monitoring routes in complex biomatrices such as blood, urine, or saliva.

Preparation of Donor Films
The materials used in this study for the preparation of the donor films are Graphene Oxide (GO) dispersed in isopropyl alcohol (1:1) at a 1 mg/mL concentration, supplied by The Graphene Box, Spain, poly (3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PE-DOT:PSS) obtained from Heraeus (Clevios PVP AI4083) with a PEDOT:PSS concentration of 1.3% w/w (weight ratio of PSS to PEDOT = 6)), and the triazene polymer, synthesized as described in [43]. All the chemicals used for the preparation of the donor films are of analytical grade and are used with no further purification.
The PEDOT:PSS: GO nanocomposite blends are prepared by adding the GO dispersion to the PEDOT:PSS solution, magnetically stirring the blend for 90 min and sonicating it for 15 min at room temperature. Two concentrations are prepared, i.e., 25% wt/wt PEDOT:PSS: GO and 50% wt/wt PEDOT:PSS: GO.
Two types of donor films are prepared, i.e., single-layer PEDOT:PSS: GO films and multilayer TP: PEDOT:PSS: GO films. Both types of donor films are prepared by spincoating.
The single-layer donor films consist of an excess amount of PEDOT:PSS: GO solution applied to the 2.5 mm × 2.5 mm quartz substrate (manual using a syringe and distributed in the cover of the coating device by centrifugation). The quartz substrate is then rotated at 500 rpm for 30 s, followed by 1500 rpm for 30 s to disperse the fluid by centrifugal force. The rotation is continued as the fluid rotates to the edges of the substrate.
The multilayer donor films are prepared in two steps. First, a triazene polymer solution (i.e., TP in chlorobenzene/cyclohexanone (1:1, w/w)) is spin coated onto the quart substrate at 2000 rpm/min, resulting in films of 100 nm thickness. Second, the PEDOT:PSS: GO solution is spread onto the quartz substrate coated with a TP film and spin coated at 1500 rpm for 30 s.
Both the single and multilayer PEDOT:PSS: GO donor films are placed on a hot plate (50 • C/30 min) to initially evaporate the solvent and solidify the coating.

Laser-Induced Forward Transfer (LIFT)
The LIFT setup used in this work consists of a pulsed ArF laser (193 nm emission wavelength, 15 ns pulse length, 1 Hz repetition rate), which is guided and imaged with an optical system onto the donor substrate to transfer PEDOT:PSS: GO micro pixels from the donor substrate to the receiving surface. A computer-controlled XYZ translation stage allows the displacement of both donor and receptor substrates with respect to the laser beam, the donor, and the receiver being placed in close contact (<1 µm). A laser pulse incident on the back side of the transparent carrier is imaged by a lens onto the donor material, yielding a spot size of 1.2 mm in diameter, and by means of shock formation and ablation, propels a small part of the donor film forward, resulting in the deposition of the PEDOT:PSS: GO pixels onto the substrate. The laser fluence is varied between 450 mJ/cm 2 and 750 mJ/cm 2 . All experiments are carried out under ambient pressure at temperatures close to room temperature.
The receiver substrates are flexible, disposable, and low-cost screen-printed (SPE) electrodes from Dropsens (ref. ITO10, Spain) with a carbon counter electrode (CE), a reference electrode (RE) made of silver, and a working electrode (WE) based on ITO. The WE consisting of 3 mm diameter ITO disk of identical commercial electrodes is modified with different PEDOT:PSS: GO pixels using a laser-induced forward transfer.

Characterization of the Laser-Transferred Materials and the As-Deposited Sensors
Topographical investigations of the transferred pixels as well as the donor films prior to ablation are carried out by means of scanning electron microscopy (SEM) with a JSM-531 Inspect S. system (Hillsboro, OR, USA) at a voltage of 20 kV. For the SEM analysis, the layers are covered using a sputter coater with 10 nm Au (Agar Scientific Ltd., Essex, UK).
The layers' surface morphology and overall roughness (quantified by the root mean square roughness) are analyzed by atomic force microscopy (AFM) (XE 100, Park Systems, Suwon, South Korea). Imaging is realized in non-contact mode, using silicon tips, in ambient conditions. The wettability of the PEDOT:PSS: GO pixels transferred onto the commercial electrodes is evaluated by contact angle measurements with a KSV CAM101 microscope equipped with a video camera. All contact-angle measurements are obtained by dispersing double-distilled water droplets with a volume of 2.5 ± 0.5 µL. Five different points are measured for every thin film, and the contact angle reported is the average of these measurements.
The electrochemical investigations are realized with an AutoLab PGSTAT302N controlled by NOVA 1.11 software. The commercial screen-printed electrode consists of an ITO working and auxiliary electrode and a silver reference electrode. In the electrode modification procedure, only the working electrode is coated by LIFT with a PEDOT:PSS: GO pixel. The connection between the potentiostat and the electrodes is made using a standard cable connector for screen-printed electrodes. All the sensors are tested for copper detection by cyclic voltammetry measurements scanning the potential from −1.4 V to 0.4 V and reversely using a scan rate of 100 mV/s. An amount of 100 ppm of Cu 2+ is added to 0.1 M acetate buffer solution (pH = 5.0) and is energetically stirred for 20 min. The amount of copper is electrochemically detected by placing 50 µL of solution over the electrode surface. The same procedure is used for testing all of the sensors. All of the tests were carried out at room temperature.

Donor Film Fabrication
The first step taken to modify electrochemical electrodes with a PEDOT:PSS: GO blend via laser-induced forward transfer is to obtain uniform and homogeneous donor films. Here, this is achieved by the spin-coating process (a scheme of the procedure is shown in Figure 1a). We carry out studies on the influence of substrate rotation speed and rotation time on the thickness and morphology of the resulting donor films. The experimental spin coating sequence chosen is 500 rpm for 30 s, followed by 1500 rpm for 30 s. The thickness of the single layer/multilayer donor films determined by at least 5 AFM measurements along scratch lines on each sample is 80 nm in the case of the PEDOT:PSS: GO coating and 180 nm for the TP/PEDOT:PSS: GO coating. Understanding the role of GO in the PEDOT:PSS matrix is very important to simplify the spin coating process and to improve the performance of the electrodes. Thus, we carried out a detailed investigation of the surface morphology of the as-deposited donor films.
The micro and nano organization/morphology of all deposited coatings is evaluated by SEM. Complete and uniform coating of the substrate surface is obtained by centrifugation, with the surface morphology of the coatings depending on the change of the dispersion's composition (Figure 1b,c). Not only the uniformity of the coatings obtained by spin coating is observed, but also the uniform distribution of the graphene oxide sheets in the composite layers with the increasing proportion of GO. We could obtain a good dispersion of the GO into PEDOT:PSS, and both concentrations, i.e., 25:75 wt.% PEDOT:PSS: GO and 50:50 wt.% PEDOT:PSS: GO lead to continuous films with some GO aggregates visible, and by increasing the GO content up to 50% wt. the presence of the aggregate increases, which can be seen in Figure 1b,c.

Laser Printing of PEDOT:PSS: GO Composites
The objective of this study is to demonstrate the fabrication of a proof-of-concept electrochemical sensor based on polymer: graphene oxide composites for copper ion monitoring. Here we focus on investigating the surface morphology of the transferred composite under different laser fluences as well as when applying an intermediate sacrificial layer as compared to the case of a direct transfer (without an intermediate layer). rotation time on the thickness and morphology of the resulting donor films. The experi-mental spin coating sequence chosen is 500 rpm for 30 s, followed by 1500 rpm for 30 s. The thickness of the single layer/multilayer donor films determined by at least 5 AFM measurements along scratch lines on each sample is 80 nm in the case of the PEDOT:PSS: GO coating and 180 nm for the TP/PEDOT:PSS: GO coating. Understanding the role of GO in the PEDOT:PSS matrix is very important to simplify the spin coating process and to improve the performance of the electrodes. Thus, we carried out a detailed investigation of the surface morphology of the as-deposited donor films. The micro and nano organization/morphology of all deposited coatings is evaluated by SEM. Complete and uniform coating of the substrate surface is obtained by centrifugation, with the surface morphology of the coatings depending on the change of the dispersion's composition (Figure 1b,c). Not only the uniformity of the coatings obtained by spin coating is observed, but also the uniform distribution of the graphene oxide sheets in the composite layers with the increasing proportion of GO. We could obtain a good dispersion of the GO into PEDOT:PSS, and both concentrations, i.e., 25:75 wt.% PEDOT:PSS: GO and 50:50 wt.% PEDOT:PSS: GO lead to continuous films with some GO aggregates visible, and by increasing the GO content up to 50% wt. the presence of the aggregate increases, which can be seen in Figure 1b,c.

Laser Printing of PEDOT:PSS: GO Composites
The objective of this study is to demonstrate the fabrication of a proof-of-concept electrochemical sensor based on polymer: graphene oxide composites for copper ion monitoring. Here we focus on investigating the surface morphology of the transferred compo-    Figure 2b.
The next step to fabricate the electrochemical sensor is to transfer the polymer: graphene oxide composites onto the commercial screen-printed electrodes. An example of a PEDOT:PSS: GO pixel transferred with a TP intermediate layer is shown in Figure 3. Each working electrode is printed with 2 PEDOT:PSS: GO pixels, placed one next to the other (thus covering approximately 32% of the surface of the working electrode).  Figure 4ac. The difference in surface topography when increasing the GO concentration is attributed to the increased wrinkling of the GO flakes, as can be seen in Figure 4. During the donor fabrication steps, both vacancies and topological defects are introduced in the carbon plane of the GO, leading to a richer texture in terms of rippling and wrinkling. The generation of these wrinkles may be attributed to the epoxy groups found on the basal plane of GO, which have the tendency to form chains on the surface of GO leading to the formation of topological defects and distortions that line up along the wrinkles path [44]. The difference in surface topography when increasing the GO concentration is attributed to the increased wrinkling of the GO flakes, as can be seen in Figure 4. During the donor fabrication steps, both vacancies and topological defects are introduced in the carbon plane of the GO, leading to a richer texture in terms of rippling and wrinkling. The generation of these wrinkles may be attributed to the epoxy groups found on the basal plane of GO, which have the tendency to form chains on the surface of GO leading to the formation of topological defects and distortions that line up along the wrinkles path [44]. The SEM images show that the transferred hybrid films using 600 mJ/cm 2 are clean and have a relatively uniform morphology without any apparent defects. This indicates that there is no sign of agglomeration of GO in composite samples and that most of the graphite sheets are dispersed homogeneously into the PEDOT:PSS polymer matrix. Therefore, the morphology of the transferred pixels suggests that the interfacial interaction between PEDOT:PSS and GO is rather strong with a very low degree of agglomeration of the GO sheets.
The SEM images show that the transferred hybrid films using 600 mJ/cm 2 are clean and have a relatively uniform morphology without any apparent defects. This indicates that there is no sign of agglomeration of GO in composite samples and that most of the graphite sheets are dispersed homogeneously into the PEDOT:PSS polymer matrix. Therefore, the morphology of the transferred pixels suggests that the interfacial interaction between PEDOT:PSS and GO is rather strong with a very low degree of agglomeration of the GO sheets. Contact-angle measurements are widely used for the characterization of the wettability of PEDOT:PSS: GO on the SPE electrodes ( Figure 5). The uncoated SPE electrode has the highest contact angle, i.e., 90 ± 2°. After transferring the PEDOT:PSS: GO blend onto the SPE electrodes, the contact angles decrease. For the 50:50 %wt. PEDOT:PSS: GO films, a minimum contact angle of approx. 70° is observed, which ensures a good coverage of the electrodes. This could be attributed to the fact that the water soluble PSS part is a good dispersant, thus allowing the improvement of the dispersion of GO in the PEDOT:PSS polymer matrix [45,46].

Electrode Modification via LIFT-Influence of Laser Fluence
Given that the scope of the paper is the demonstration of proof-of-concept electrochemical sensors for the detection of copper ions, we carried out functionality tests of the Contact-angle measurements are widely used for the characterization of the wettability of PEDOT:PSS: GO on the SPE electrodes ( Figure 5). The uncoated SPE electrode has the highest contact angle, i.e., 90 ± 2 • . After transferring the PEDOT:PSS: GO blend onto the SPE electrodes, the contact angles decrease. For the 50:50 wt.% PEDOT:PSS: GO films, a minimum contact angle of approx. 70 • is observed, which ensures a good coverage of the electrodes. This could be attributed to the fact that the water soluble PSS part is a good dispersant, thus allowing the improvement of the dispersion of GO in the PEDOT:PSS polymer matrix [45,46].
The SEM images show that the transferred hybrid films using 600 mJ/cm 2 are clean and have a relatively uniform morphology without any apparent defects. This indicates that there is no sign of agglomeration of GO in composite samples and that most of the graphite sheets are dispersed homogeneously into the PEDOT:PSS polymer matrix. Therefore, the morphology of the transferred pixels suggests that the interfacial interaction between PEDOT:PSS and GO is rather strong with a very low degree of agglomeration of the GO sheets. Contact-angle measurements are widely used for the characterization of the wettability of PEDOT:PSS: GO on the SPE electrodes ( Figure 5). The uncoated SPE electrode has the highest contact angle, i.e., 90 ± 2°. After transferring the PEDOT:PSS: GO blend onto the SPE electrodes, the contact angles decrease. For the 50:50 %wt. PEDOT:PSS: GO films, a minimum contact angle of approx. 70° is observed, which ensures a good coverage of the electrodes. This could be attributed to the fact that the water soluble PSS part is a good dispersant, thus allowing the improvement of the dispersion of GO in the PEDOT:PSS polymer matrix [45,46].

Electrode Modification via LIFT-Influence of Laser Fluence
Given that the scope of the paper is the demonstration of proof-of-concept electrochemical sensors for the detection of copper ions, we carried out functionality tests of the

Electrode Modification via LIFT-Influence of Laser Fluence
Given that the scope of the paper is the demonstration of proof-of-concept electrochemical sensors for the detection of copper ions, we carried out functionality tests of the LIFT-fabricated PEDOT:PSS: GO working electrodes. The functionality tests are carried out by evaluating the response of the laser printed electrodes in different experimental conditions at 100 ppm Cu 2+ . In particular, the experimental conditions investigated are the influence of the laser fluence, the polymer: graphene oxide ratio, and the addition of an intermediate triazene polymer layer for printing the polymer: graphene oxide composite. The cyclic voltammogram obtained for the utilization of bare (untreated) flexible SPE working electrode for the electrochemical detection of 100 ppm Cu 2+ in 0.1 M acetate buffer solution is presented in Figure 6a. During the forward scan, at around −430 mV, a single cathodic peak (I pc ), correlated to the Cu 2+ reduction via two-electron transfer reaction (Cu 2+ + 2e − → Cu 0 ) is obtained. Taking into account that the reaction medium has a pH = 5, the bivalent copper is directly reduced to metallic copper [47]. For the reverse scan, the oxidation of metallic copper through a one-step oxidation reaction is observed with an anodic peak (I pa ) at +26 mV. For all untreated and modified electrodes, data from current vs. potential plots are gathered in Table 1.  Figure 6a. During the forward scan, at around −430 mV, a single cathodic peak (Ipc), correlated to the Cu 2+ reduction via two-electron transfer reaction (Cu 2+ + 2e −  Cu 0 ) is obtained. Taking into account that the reaction medium has a pH = 5, the bivalent copper is directly reduced to metallic copper [47]. For the reverse scan, the oxidation of metallic copper through a one-step oxidation reaction is observed with an anodic peak (Ipa) at +26 mV. For all untreated and modified electrodes, data from current vs. potential plots are gathered in Table 1. The current ratio ipa/ipc is another important parameter which can be extracted from cyclic voltammograms. It gives information about the chemical stability of the electrochemically generated product and its stability is demonstrated by a peak current ratio of unity. It also gives information about the reversibility of the system. The reversibility of a redox system is correlated to the "rapidity" of the analyte to exchange electrons at the electrode. For completely Nernstian systems (reversible redox systems), the electron transfer occurs quickly without significant thermodynamic barriers (electron transfer rate ≫ mass transfer rate of diffusion). In addition, the theoretical value for the separation between the two peak potentials, ΔEp is equal to 59/n mV, regardless of scan rate, for a reversible process. Moreover, the ratio of peak currents for reversible systems is equal to unity (ipa/ipc = 1) [47,48]. When studying the influence of the laser fluence on the printed PEDOT:PSS: GO blend, it has been found that the electrochemical properties of PEDOT:PSS: GO sensors fabricated at different laser fluences are quite similar, the positions of anodic and cathodic peak potentials being slightly shifted between 110-150 mV and 470-480 mV, respectively (Figure 6b). For samples 1 and 2, the values of I pa are smaller, while for sample 3 the value of I pa is similar compared to the commercial electrode, while I pc values increase up to 63% (for sample 3, i.e., the PEDOT:PSS: GO pixel transferred at 450 mJ/cm 2 ) as compared to the bare sensor.
The current ratio i pa /i pc is another important parameter which can be extracted from cyclic voltammograms. It gives information about the chemical stability of the electrochemically generated product and its stability is demonstrated by a peak current ratio of unity. It also gives information about the reversibility of the system. The reversibility of a redox system is correlated to the "rapidity" of the analyte to exchange electrons at the electrode. For completely Nernstian systems (reversible redox systems), the electron transfer occurs quickly without significant thermodynamic barriers (electron transfer rate mass transfer rate of diffusion). In addition, the theoretical value for the separation between the two peak potentials, ∆E p is equal to 59/n mV, regardless of scan rate, for a reversible process. Moreover, the ratio of peak currents for reversible systems is equal to unity (i pa /i pc = 1) [47,48]. The increase of the i pa /i pc ratio can be correlated to a lower reversibility of the system. All fabricated working electrodes through the LIFT technique show higher reversibility than the commercial electrode, except for the sensor prepared with 50:50 wt.% PEDOT:PSS: GO, without TP, at a laser fluence of 600 mJ/cm 2 , which shows a i pa /i pc = 5.01. However, in our case, for all tested sensors, both oxidation and reduction peaks are obtained. The peak potential separation (∆E p ) is between 580-630 mV and the ratio of the oxidation peak current to the reduction peak current is about 1.97-2.79, values similar to those reported by Shaikh et. al. [49], but higher compared to those reported by Haque et. al. [50]. The values of the separation between the two peak potentials, ∆E p is different than 59/n mV and i pa /i pc > 1, so we can assume that the redox system of copper at the graphene composite is a quasi-reversible one.

Electrode Modification via LIFT-Influence of Polymer: Graphene Oxide Ratio
The polymer: graphene oxide ratio in the transferred pixel plays an important role on the electrochemical response of the PEDOT:PSS: GO modified SPE sensors (Figure 7). For example, when a 25:75 wt.% PEDOT:PSS: GO (sample 2) ratio is utilized for sensors fabrica-tion, the resulting device shows an intense oxidation peak with a value of I pa = 40.66 µA and I pc = 14.59 µA. By increasing the polymer concentration to a ratio of 50:50 wt.% PE-DOT:PSS: GO, a decrease of both peak currents is observed. At the same time, the anodic peak potential is shifted to more negative potentials, while the cathodic peak potential is shifted to positive potentials. As can be observed in Table 1, the peak potential separation decreases with polymer concentration, while the ratio of the oxidation peak current to the reduction peak current is almost twice as high for the sensor that is richer in the polymer.
fabrication, the resulting device shows an intense oxidation pe µA and Ipc = 14.59 µA. By increasing the polymer concentrat PEDOT:PSS: GO, a decrease of both peak currents is observed odic peak potential is shifted to more negative potentials, whi tial is shifted to positive potentials. As can be observed in Tab aration decreases with polymer concentration, while the ratio rent to the reduction peak current is almost twice as high for the polymer.  Figure 8, for the sample printed with triazene, the anodic c µA (the PEDOT:PSS: GO electrode printed at a laser fluence sample shows a slight increase of the Ipc compared to the va printed without triazene and a decrease of the oxidation pea peak current. In addition, we have found that the values of sensors printed with a triazene polymer intermediate layer a SnO2 and Pd-SnO2 based sensors reported in [19]. These resu with results reported in the literature [42], which also show th polymer layer is an important parameter for obtaining a regu over, the triazene polymer, as an intermediate layer, absorbs t sequently decomposes into gaseous fragments which are use The addition of an intermediate triazene polymer layer in the laser transfer process has a major impact on the electrochemical behavior of the final device. As can be observed in Figure 8, for the sample printed with triazene, the anodic current increases up to 48.84 µA (the PEDOT:PSS: GO electrode printed at a laser fluence of 600 mJ/cm 2 ). The same sample shows a slight increase of the I pc compared to the value recorded by the sensor printed without triazene and a decrease of the oxidation peak current to the reduction peak current. In addition, we have found that the values of I pa and I pc obtained for the sensors printed with a triazene polymer intermediate layer are higher compared to the SnO 2 and Pd-SnO 2 based sensors reported in [19]. These results are in good agreement with results reported in the literature [42], which also show that the addition of a triazene polymer layer is an important parameter for obtaining a regular, "clean" transfer. Moreover, the triazene polymer, as an intermediate layer, absorbs the laser radiation and subsequently decomposes into gaseous fragments which are used to transform the energy into a required mechanical push. Thus, we could assume that the presence of the TP layer in the donor improves the interfacial adhesion of the pixel to the SPE electrode, which is in turn a basic factor to obtain a good working electrode in electrochemical sensor applications. An interesting trend is observed for triazene based sensors fabricated at different laser fluences (Figure 9). Both Ipa and Ipc decreases with the increase of the laser fluence, and peak potentials are shifted toward negative potentials (for Epa) and positive potentials (for Epc), respectively.
It can be concluded that the functionality tests carried out with the LIFT modified commercial SPE electrodes, i.e., with a PEDOT:PSS: GO blend are feasible for the detection of copper ions.

Conclusions
In this work, a laser-induced forward transfer (LIFT) method was optimized for printing polymer: graphene oxide (GO) pixels aiming at the fabrication of electrochemical sensors for the detection of copper ions. The pixels were produced by transferring poly (3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS): GO composite on the working electrode of a commercial electrochemical flexible sensor. Our approach is simple and flexible due to the fact that PEDOT:PSS:GO composites at different ratios can be easily deposited by LIFT on various supports, including flat and non-conformal plastics, and there is no need for prior surface functionalization. An interesting trend is observed for triazene based sensors fabricated at different laser fluences ( Figure 9). Both I pa and I pc decreases with the increase of the laser fluence, and peak potentials are shifted toward negative potentials (for E pa ) and positive potentials (for E pc ), respectively. An interesting trend is observed for triazene based sensors fabricated at different laser fluences (Figure 9). Both Ipa and Ipc decreases with the increase of the laser fluence, and peak potentials are shifted toward negative potentials (for Epa) and positive potentials (for Epc), respectively.
It can be concluded that the functionality tests carried out with the LIFT modified commercial SPE electrodes, i.e., with a PEDOT:PSS: GO blend are feasible for the detection of copper ions.

Conclusions
In this work, a laser-induced forward transfer (LIFT) method was optimized for printing polymer: graphene oxide (GO) pixels aiming at the fabrication of electrochemical sensors for the detection of copper ions. The pixels were produced by transferring poly (3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS): GO composite on the working electrode of a commercial electrochemical flexible sensor. Our approach is simple and flexible due to the fact that PEDOT:PSS:GO composites at different ratios can be easily deposited by LIFT on various supports, including flat and non-conformal plastics, and there is no need for prior surface functionalization. It can be concluded that the functionality tests carried out with the LIFT modified commercial SPE electrodes, i.e., with a PEDOT:PSS: GO blend are feasible for the detection of copper ions.

Conclusions
In this work, a laser-induced forward transfer (LIFT) method was optimized for printing polymer: graphene oxide (GO) pixels aiming at the fabrication of electrochemical sensors for the detection of copper ions. The pixels were produced by transferring poly (3,4ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS): GO composite on the working electrode of a commercial electrochemical flexible sensor. Our approach is simple and flexible due to the fact that PEDOT:PSS:GO composites at different ratios can be easily deposited by LIFT on various supports, including flat and non-conformal plastics, and there is no need for prior surface functionalization.
Surface characterization of the transferred features using atomic force microscopy (AFM) and scanning electron microscopy (SEM) corroborated that the PEDOT:PSS:GO composite was transferred in a "clean" and regular manner, with a high resolution on the surface of the working electrode of a commercial electrochemical flexible sensor. Furthermore, by applying the PEDOT:PSS: GO composite on the working electrode, a decrease of the contact angle can be measured, i.e., from 90 • to 70 • for the SPE printed with a 50:50 wt.% PEDOT:PSS: GO pixel.
We were able to show for the first time the transfer of polymer: graphene oxide composites as thin-pixels onto flexible, working electrodes and prove their functionality by testing them against copper ions. As copper ion detection has received great attention in recent years, we intend to expand our studies and investigate the performance of our modified electrodes towards different concentrations of copper ions further. In addition, it will be interesting to determine the minimum quantity of composite on the surface of the commercial electrode which renders the highest effect towards copper ion determination. We believe that this study, i.e., the demonstration of modifying working electrodes via laser-based methods for the determination of copper ions, might represent a solution for potential applications in diagnostic tools for point of care testing.

Data Availability Statement:
The data used to support the findings of this study are available from the corresponding author upon request.